Generally, a high-power underground electrical transmission line is designed to operate at medium voltages (typically from 10 to 60 kV) and high voltages (typically higher than 60 kV), and currents of the order of hundreds of amperes (typically from 500 to 2000 A). The electrical power carried in these lines can reach values of the order of hundreds of MVA, typically 400 MVA. Normally, the current carried is an alternating current at low frequency, in other words generally below 400 Hz, and typically at 50-60 Hz. In general, these lines are used for transferring power from electrical power stations to urban centres, over distances of the order of tens of km (normally 10-100 km),
Typically, electrical power transmission lines are three-phase lines comprising three cables buried in a trench at a depth of 1-1.5 m. In the space immediately surrounding the cables, the magnetic field H can reach relatively high values and, at ground level (i.e. at 1-1.5 m distance from the line), a magnetic induction as high as 20-60 μT (depending on the geometrical arrangement of the cables with respect to each other) can be measured.
To avoid possible biological effects due to exposure to magnetic fields of this size generated by low-frequency (50 Hz) sources, a “safety threshold” is considered, below which the probability of biological damage can be reduced to a minimum, if not eliminated. A threshold of magnetic induction on which some national legislation is tending to become harmonized, is 0.2 μT, a value which is approximately 100 times smaller than that generated by an unshielded three phase line as indicated above.